 In this video we are going to look at a laboratory test called Lucas test which is used to distinguish between primary, secondary and tertiary alcohols. So what broadly happens here is that we react an alcohol with a Lucas reagent which is nothing but a mixture of hydrochloric acid and anhydrous zinc chloride to give us the corresponding alkyl chloride. Now this reaction proceeds via SN1 mechanism when we have a secondary or a tertiary alcohol and SN2 mechanism when we have a primary alcohol. Now this is obvious right because SN1 reaction of a primary alcohol with Lucas reagent will not take place at all because the reaction proceeds via a carbocation intermediate and when we have a primary alcohol the SN1 reaction would have to proceed via a primary carbocation intermediate and we know that primary carbocations are extremely unstable. So this is why SN1 reaction of primary alcohol with Lucas reagent does not take place. But SN2 reaction of primary alcohol with Lucas reagent does proceed. As you can see here the lone pair of electrons on the oxygen atom coordinates strongly with zinc chloride. Now in the usual case when we treat an alcohol with an acid the OH group gets protonated. But the advantage of using ZnCl2 here is that zinc chloride coordinates much more strongly with the oxygen atom as compared to proton and as a result this becomes a much better leaving group. Now even though the primary alcohol does indeed react with Lucas reagent via SN2 mechanism this reaction is still very slow. In other words this SN2 reaction is still much slower than the SN1 reaction of a secondary or tertiary alcohol. So basically the reaction of primary alcohol with Lucas reagent is very slow. Now if you look at the SN1 reaction of secondary alcohol with Lucas reagent you can see that the first step is essentially the coordination of oxygen atom with zinc chloride. So here it forms a very good leaving group and as the leaving group leaves we get a secondary carbocation here. Now our nucleophile which is a chloride ion attacks this secondary carbocation giving us a final alkyl chloride. The SN1 reaction of a tertiary alcohol also proceeds the same way where we get a tertiary carbocation which is then attacked by a chloride ion giving us a tertiary alkyl chloride. Alright now we understood what exactly happens in the Lucas reaction but how are we still able to figure out or distinguish between the different types of alcohols? Well that is through their reactivity time. You see the primary secondary and tertiary alcohols react at very predictable rates with our Lucas reagent and we can use these rates and the time of reaction to distinguish between the different types of alcohols. Now when we add the Lucas reagent to our alcohol it first forms a single homogeneous phase and once the reaction is complete when the alcohol has completely reacted with Lucas reagent to give us alkyl chloride the starting clear solution now becomes a turbid solution that is it looks more like a suspension. Now if you look at the time of reaction between the different types of alcohols and the Lucas reagent you can see that tertiary alcohols react almost immediately with Lucas reagent and forms a turbid solution under one minute. This is because tertiary carbocations are much more stable and the reaction or the turbid solution appears almost instantaneously. Now in the case of secondary alcohols the time taken for the reaction or the turbid solution to form is usually about 1-5 minutes because secondary carbocations are less stable than the tertiary carbocations. Now primary alcohols as we already discussed react very slowly with Lucas reagent. In fact with the primary alcohol the reaction can take from over 6 minutes to even several days. So this is the basis on which we can differentiate between the different types of alcohols.